It is known to construct the heat exchangers for gas fired, hot air furnaces from a pair of metal plates or sheets secured in face to face relationship to form a multi-pass flow passage for the hot combustion gas of the furnace. This type of heat exchanger is commonly referred to as a multi-pass clamshell heat exchanger. Typically, the multi-pass flow passage includes an inlet section an outlet section, and one or more passes connecting the inlet and outlet sections. The inlet section receives hot combustion gases from a burner, such as an inshot burner, and provides a combustion zone for the gases. The outlet section communicates with an induction draft blower or power vent which serves to draw the hot combustion gases through the multi-pass flow passage of the heat exchanger. As the combustion gas flows through the heat exchanger, it cools and becomes more dense. To maintain high gas velocity, it is known to decrease the flow area of the heat exchanger from pass to pass. It is common for a gas fired furnace to include a plurality of such clamshell heat exchangers, spaced apart in a parallel array to define air flow paths so that heat may be transferred from the hot combustion gas through the plates of the heat exchangers to the air flowing through the furnace. Examples of such clamshell heat exchangers are shown in U.S. Pat. No. 5,359,989 issued Nov. 1, 1994 to Chase et al., and U.S. Pat. No. 4,467,780 issued Aug. 28, 1984 to Ripka, the complete disclosures of which are incorporated herein by reference.
One problem commonly found in known clamshell heat exchangers are the relatively sharp angle bends that result from the formation of the hot gas combustion flow passage in the sheet metal. For example, the clamshell heat exchanger (12) in the U.S. Pat. No. 5,359,989 requires four relatively sharp angle bends for each passage (24a, 25a-c, 26a-c, and 27a-c). Such sharp angle bends produce localized material stretching that can reduce or damage anti-corrosion coatings on the surface of the material, thereby increasing the likelihood of premature corrosion failure.
Further, while many known clamshell heat exchangers perform satisfactorily, there is a continuing desire to produce more compact and efficient furnaces by decreasing the size of the heat exchangers and/or increasing the heat exchanger's performance characteristics.